Electric heating device and method of producing the same



June 24, 1930. E, N. LIGHTFOOT 1,767,034

ELECTRIC HEATING DEVICE AND METHOD OF PRODUCING THE SAME Filed Feb. 2, 1925 lNlliYY'O/V Patented June 24, 1930 FATE OFFICE EDWIN IN. LIGHTFOOT, F MILWAUKEE, WISCONSIN, ASSIGNOR, BY MESNE ASSIGN- MEETS, TO CUTLER-HAMMER, ENG, 0F MILWAUKEE, WISCONSIN, A CORPORATION OF DELAWARE ELECTRIC HEATING DEVICE AND METHOD OF PRODUCING- THE SAME Application filed. February 2, 1925. Serial No. 6,170.

This invention relates to electric heating devices and to a method of producing the same.

While it is to be understood that the expression electric heating devices as herein employed embraces rheostats, the invention is particularly applicable to devices for heating purposes, and, more especially, to devices having surfaces of considerable area to be heated substantially uniformly throughout.

One of the most satisfactory heaters of this type utilizes a resistor enclosed in a tube and insulated therefrom by a filling of tightly packed granular or powdered magnesium oxide, the resistor unit being cast in the body to be heated or otherwise integral- 1y united therewith. This type of heater has proven very advantageous because of g the thorough insulation of the resistor and the high degree of heat conductivity of the insulating filling. On the other hand, the

employment of a separately formed enclosed resistor involves considerable expense and presents difliculties in uniting the resistor unit and the object to be heated for the best results.

The present invention has among its objects to provide a comparable heater which may be constructed without employment of a separately formed enclosed resistor.

More specifically, the invention has among its objects to provide for mounting the bare resistor on or within the body or object to be heated and for insulating such resistor .by the same material that has been found so advantageous in the enclosed units of the type aforementioned, such material being compacted to a degree equal to or greater than that of such enclosed units.

Another object is to enable the production of the proposed heating device in a convenient and expeditious manner and at a reasonable cost; W Other objects and advantages of the invention will hereinafter appear.

The invention will now be further described, reference being had to the accompanying drawings disclosing certain embodiments thereof.

a. I t

V In the drawings:

Figure l is a plan view of a sad iron base with the resistor placed thereon;

Fig. 2 is a similar view of the base slightly modified;

Figs. 3 to 6 are views illustrative of the processes hereinafter set forth, each view showing a grooved portion of the base in cross sectlon;

Fig. 7 is a sectional view through the base adjacent one terminal, and

Figs. 8 and 9 are views similar to Figs. 3 to 6, but illustrative of a modified process.

Referring to Fig. 1, the sad iron base 1 shown therein is provided with a tortuous groove 2 to receive a helical resistor 3, said groove being so formed that the resistor is distributed to heat the base substantially uniformly throughout. The groove 2 has an insulating lining 4 which preferably is formed of tightly packed powdered insulating material, although, as will appear, the lining might comprise insulation in other forms, as, for example, an enamel.

Where the lining is formed of powdered insulation it is preferred to employ commercially pure magnesium oxide, experiments having established that if a quantity of such material is placed in the groove as depicted in Fig. 3 and then subjected to the pressure of a forming die or ram 5, a very solid lining of-the cross section shown in Fig. 4 may be obtained. Preferably the magnesium oxide is moistened prior to such treatment, although due to inherent properties of such a material no binder is necessary, and the lining formed as described with or without moistening will remain intact while the resistor is arranged therein. In fact, the magnesium oxide when packed in the groove under a pressure of between five to ten tons per square inch of oxide surface has been found machinable Without impairment thereof as a lining, and, accordingly, instead of forming the lining as described, the groove may be filled with the pressed material which thereafter may be machined to receive the helical resistor.

Moreover, the lining when formed in either manner aforedescribed may be further hardened if so desired by subjecting heater is placed in service, but as the con versi n into hydroxide produces a stone like mass this step may be desirable to further insure protection of the lining during the subsequent steps of manufacture.

After lining the groove the resistor is ar anged therein with its adjacent turns separated to the desired extent, it, of course, being understood that any other preferred form of resistor may be substituted for the helical form illustrated. Preferably the resistor is provided with terminals which may assume various different forms. As illustrated, the extremities 6 of the resistor are led into terminal members 7 which are preferably swaged onto the resistor for electrical contact. The terminal members 7 as shown in Fig. 7 are L-shaped and are seated in the roove 4, the upright portions of sa d mem ers passin through lava or other su table insulating bushings seated in the base 1 and adapted to be retained therein by a plate 8 to be secured to base 1 as by a screw 9 after further insulating of the resistor as now to be described.

When the resistor has been placed in the groove as described a further quantity of powdered magnesium oxide 18 introduced into the groove as depicted in Fig. 5 and compacted under the pressure of a rain 10 to embed the resistor as shown in Fig. 6.

The magnesium oxide may be thus introduced in either a. dry or moistened state and subjected to any suitable pressure to force the same between the turns of the resistor to fill the space within the latter and to produce-a hard dense mass in which the resistor is firmly anchored, said mass being by the same treatment pressed into intimate contact with the walls of the grooves and thereby anchored in place.

Thus when the insulating mass is dried out, if previously moistened, the resistor is thoroughly insulated from the base while the compacted mass of insulation links the resistor and base in an efiicient thermal relation. Powdered magnesium oxide, as is well understood, is both a goodelectrical insulator and heat conductor,;and these properties coupled with those adapting the same 1 to solid mass formation under pressure render employment of such material very advantageous. However, it is to be understood that magnesium oxide is specified only as a preferable material.

The compacted filling may be left exposed or the same may be protected as shown in Fig. 6 by a covering 11 of cement, rigid thereon in the 'upon the mass may be insulating material or metal fixed to the slotted face of the heated body.

Referring to Fig. 2, the base 1 instead of being provided with a groove is merely provided with a continuous outer flange 12 and a plurality of upstanding lugs 13 affording anchorage for the insulating filling which is introduced into all recesses and formed with a continuous groove as in Fig. 1. Such a construction may be preferable in some instances especially where a more tortuous or complex wire passage is desired, as it facilitates formation of the base and renders of less importance accurate interfittin of the ram 5 and the base. Moreover, t is construction requires less care in placing the groove if machined in the insulating mass, although, of course, a given clearance with all lugs should be provided for, es ecially within the bends of the resistor. W ere the resistor is stretched in a straight line between any two points, tensioning thereof will tend to retain it in a proper osition, but an insulating wall of proper t ickness between it and adjacent lugs is nevertheless obviously desirable.

In practice it may be desirable to form the lugs 13, Fig. 2, or the walls of the groove, Fig. 1, as the case may be, to prevent accumulation of insulating material it is deemed' unnecessary to illustrate any of the various obvious ways of accomplishing this result. Also, in sometimes be expedient to dispense with the lining of the resistor'receiving recess and to support the resistor at spaced points on suitable insulating inserts of small dimensions for retaining the resistor in proper position to be embedded in the subsequently formed insulating mass, but illustration of this modification is likewise deemed unnecessary.

Referring to Figs 8 and 9, the same illustrate still another substitute for the lining aforedescribed. As disclosed in these figures, the groove of base 1 may be filled with a plastic mass of insulating material and the resistor supported on ram 14 whereby said resistor may be forced into the plastic mass and retained therein in its roper location upon retraction of the ram. Thereagain subjected to pressure to complete the inclosure of the resistor, more insulating material being added if necessa By forming the ram 14 as illustrated t e material displaced b insertion of the resistormay be retained adjacent the groove for reinsertion therein and subserplient com acting by a ram similar to that s own in ig. 5. 1

As will be understood, it is desirable to dry out the finished product before the 'same is put into service, and this may be accomplished in any preferred manner, as for expractice it may ample, by placing the device in a drying oven.

What I claim as new and desire to secure by Letters Patent is:

1. The method of equipping a body to' be heated with a resistor for heating thereof, which comprises applying to the body a quantity of magnesium oxide particles in which the resistor is embedded and then subjecting the mass of magnesium oxide per se to a pressure for effecting cohesion of its particles and anchorage to said body.

2. The method of securing a resistor to a body to be heated which comprises embedding the resistor in a quantity of divided magnesium oxide applied to the body and then solidifying the mass of magnesium oxide and engaging the same with the body by pressure applied directly to the mass.

3. The method of equipping a body to be heated with a heatingv resistor which comprises embedding the resistor in a quantity of divided insulating material within an open groove in the body and then by applied pressure alone eflecting a hard mass formation of said material which is self attaching to the body.-

4. The method of securing a resistor to a body to be heated thereby which comprises equipping the body with insulating supports for the resistor, mounting the resistor on such supports, surrounding the resistor with a disintegrated insulating material and then applying pressure directl to the insulatin 9. An electric heating device comprising a body to be heated, a mass of insulating material supported directly thereby and a resistor surrounded and supported by said mass, the latter comprising finely divided magnesium oxide converted in situ into a coherent body by pressure applied directly thereto.

10. An electric heating device comprising a body to be heated, a resistor support preformed thereon and comprising compacted magnesium oxide, a resistor mounted on said support and an enclosure for said resistor also comprising finely divided magnesium oxide compacted in situ by pressure applied directly thereto to secure said resistor to said body.

In witness whereof, I have hereunto subscribed my name.

EDWIN N. LIGHTFOOT.

material to alone efi'ect co esion of the part1- cles of insulating material to produce a hard mass which is self attaching to said body.

5. The method of forming a heating device which comprises applying to the body to be heated a quantity of magnesium oxide in a divided state, subjectin such magnesium oxide ,to pressure and orming in the resulting mass a groove, placing a resistor within such groove, surrounding said resistor with additional magnesium oxide in a like state and again subjecting the mass to pressure to effect cohesion of its particles and anchorage to said body.

6. The method of preparing for reception of an electrical resistor, a body to be heated thereby which comprises applying to the body a quantity of insulating material in a divided state, subjecting the insulating material'to pressure alone to provide a hard mass, and then grooving the mass to receive the resistor.

7 The method of preparing a body to be heated for receivin a heating resistor which comprises compactlng thereon a quantity of finely divided magnesium oxide to form a resistor support and then converting the compacted magnesium oxide to magnesium hydroxide.

8. An electric heating device comprising a body to be heated, said body having a 

